The phosphoric acid solutions prepared by the wet process contain several impurities, in particular magnesium of which the proportion depends on the original mineral; this element is an exceedingly bothersome impurity because in the long run causing precipitates that turn into sediments in the tanks and in the plant conduits: beyond the practical difficulties thus raised, these sediments or deposits entail more than trivial losses of P.sub.2 O.sub.5. Also, the Mg impurities create difficulties, well known to the experts, in most subsequent processing of phosphoric acid (manufacture of fertilizers, of superphosphoric acid).
Furthermore the phosphoric acid solutions prepared by the wet process also contain most of the time aluminum-based and/or fluorine-based impurities which, though somewhat less interfering than the above impurities, nevertheless may entail parasitic precipitations.
The quantities of phosphoric acid produced by the wet process are very large (37 million tons of phosphoric acid were produced in the world in 1975, of which 84% by the wet process), and much research has been carried out toward its purification which presently is done by three methods:
extraction by solvent, which is a recent process being developed, with the drawback that complex equipment is needed that increases substantially capital expenditures, PA1 exhaustion on ion exchange resins, which is an older process with the drawback of being very costly and also requiring complex equipment, PA1 precipitation in the form of insoluble complexes and separation by filtration or decantation.
The object of the invention is a process of the latter kind.
Presently essentially two chemical reactions are known which are applied to precipitate the magnesium and possibly also aluminum and fluorine.
French patent No. 2,065,265 (Allied Chemical Corp.) describes a process for eliminating magnesium, aluminum and fluorine in the form of a magnesium-aluminum phosphate fluoride precipitate obtained by adding fluoride and aluminum ions to be phosphoric acid in suitable amounts to obtain such a compound.
French patent No. 2,419,254 (Agrico Chemical Co.) describes a separation process for the magnesium and aluminum impurities by precipitating them in the form of MgAl.sub.2 F.sub.8 --MgAlF.sub.5 by adding fluoride ions and where called for aluminum ions to the phosphoric acid in suitable proportions to obtain these compounds.
These two patents should be consulted for further details on the reactions taking place and the operating conditions; these reactions offer the advantage of requiring only very little additional equipment in phosphoric acid production plants. However the precipitates obtained in both cases are in gelatinous form and entail a relatively delicate and long physical separation (filtration or decantation); moreover, the gelatinous structure captures and adsorbs the phosphoric acid and causes a significant reduction in P.sub.2 O.sub.5 content and therefore a reduction in overall yields of manufactured acid. Again, these two precipitation reactions are slow and require several tens of hours for effective precipitation, whereby the production cycle of the phosphoric acid is lengthened by as much and the volume of the decantation tanks required is increased.
The present invention proposes to provide a new process for separating magnesium impurities and where called for aluminum and fluorine impurities, from phosphoric acid by carrying out a novel precipitation reaction resulting in forming a new insoluble complex.
The invention seeks to benefit from the advantages of this precipitation process (ease of implementation and low equipment cost) while also eliminating the above discussed drawbacks of the known procedures of this type.
An object of the invention is in particular to induce formation of a crystalline precipitate comprising crystals larger than 0.1 microns which are easily separated by filtration o decantation and which only very slightly adsorb phosphoric acid.
Another object is to reduce the precipitation times for decreasing them to on the order of several tens of minutes to several hours.